Hydrogels are hydrophilic polymer networks produced from reactions of one or more monomers or by association bonds between chains that can absorb from at least 20% to up to thousands of times their dry weight in water. Hydrogels may be chemically stable or they may disintegrate and dissolve with time. Hydrogels may be classified as either physical or chemical. Physical hydrogels have networks held together by molecular entanglements and/or secondary forces such as hydrogen bonding, van der Waals interactions, ionic or hydrophobic forces. Physical hydrogels are not homogeneous due to regions of high crosslinking density and low water swelling, called clusters, dispersed within low crosslinking density and high water swelling, or hydrophobic or ionic domains that create inhomogeneities. Chemical hydrogels are covalently crosslinked networks and may be generated by the crosslinking of water-soluble polymers, or by converting hydrophobic polymers to hydrophilic polymers. Chemical hydrogels are also not homogeneous due to clusters of molecular entanglements. Chain loops and free chain ends also produce network defects in both physical and chemical hydrogels, and they do not contribute to the permanent network elasticity.
Electroactive hydrogels are those hydrogels prepared using a polyelectrolyte polymer and whose shape and/or dimensions are altered upon pH and/or modest electric field change. For example, in a charged polyelectrolyte polymer, the polymer chains are chemically linked to one another through cross linking sites and swollen by solvent molecules, such as water that “ionize” the acid or salt groups along the polymer backbone to yield mobile hydrated ions (e.g., cations) and immobile anions attached to the polymer backbone. It is the mobility of hydrated ions, afforded by swelling the hydrogel with a suitable solvent, that leads to an electroactive response.
Common applications for hydrogels include use in super-absorbant materials, contact lenses and cosmetics. In addition, hydrogel materials have been used for drug delivery and to replace or reconstruct soft tissues. However, the utility of such hydrogels has been hindered due to limitations in the elasticity, force generation abilities and responsiveness of the prior art hydrogels.
Accordingly, there is a need in the art for improved electroactive hydrogels and methods of making such hydrogels.